In motor vehicles, a heat accumulator for the heat which is contained in the coolant, which the heat accumulator is embodied, for example, as a coolant accumulator or latent heat accumulator and can hold the heat stored therein for, for example, at least 24 hours permits savings in fuel consumption and correspondingly lower emissions of pollutants. The heat which is stored in the heat accumulator during travel and/or at the end of travel is used to return the internal combustion engine as quick as possible to its operating temperature when it is activated again, which significantly reduces the frictional losses in the engine and permits a fuel/air mixture to be generated which is optimum in terms of fuel consumption. The stored heat can also be used to heat quickly other vehicle components such as, for example, the vehicle battery and/or the passenger compartment of the vehicle.
DE 195 12 821 A1 discloses a method for operating a heat accumulator in a motor vehicle, in which the heat accumulator is charged with the available heat at the latest when the ignition is switched off. This contributes to the heat contained in the coolant being charged into the accumulator and discharged again later with the greatest possible efficiency.
DE 10 2012 006 632 A1 discloses a method for operating a heat accumulator in a motor vehicle, in which the thermal coupling between the heat accumulator and the heat sinks which are fed thereby occurs as a function of a distance which is still to be covered by the vehicle.
A motor vehicle heat accumulator is typically charged in small increments as soon as the coolant has reached relatively high temperatures or when excessive heat energy is available. This is the case when the thermostat temperature of the engine cooling system at which the coolant is permitted to circulate through the radiator is reached or almost reached.
During short journeys or at low ambient temperatures, the coolant temperature possibly does not reach more than 70° C. As long as the coolant temperature is still relatively low, it is unfavorable to charge the heat accumulator because all the heat is required for the further heating of the internal combustion engine and, if appropriate, for heating the passenger compartment of the vehicle.
If, in such a case, the internal combustion engine is deactivated by switching off the ignition and the vehicle is left in a stationary state, it is also possibly uneconomical now to charge the heat contained in the coolant into the accumulator. This depends on the period of time for which the vehicle remains non-operational. For relatively short stationary times of, for example, less than an hour, heat accumulation is uneconomical because the losses of the heat exchange process are greater than the benefit of the heat accumulation.
It would be possible to consider allowing the heat exchange process to start automatically only a certain time after the ignition has been switched off, but up to then heat would be lost into the surroundings, with the result that the heat accumulation would have less benefit.